sdk tests: run the wasmtime CLI without a disk cache; show its stderr on EOF

The Nix sandbox has no writable home folder, so wasmtime run failed to
create its cache folder.

sdk/tests/wasi.rs

@@ -32,7 +32,14 @@ impl Process {
     fn read(&mut self) -> Value {
         loop {
             let mut s = String::new();
-            assert_ne!(self.output.read_line(&mut s).unwrap(), 0, "unexpected EOF");
+            if self.output.read_line(&mut s).unwrap() == 0 {
+                let mut stderr = String::new();
+                let _ = std::io::Read::read_to_string(
+                    self.child.stderr.as_mut().unwrap(),
+                    &mut stderr,
+                );
+                panic!("unexpected EOF from wasmtime. Its stderr:\n{stderr}");
+            }
             if let Ok(v) = sausage_plugin::serde_json::from_str::<Value>(&s)
                 && v["jsonrpc"] == "2.0"
             {
@@ -51,6 +58,9 @@ fn start(wasm: &Path, dir: &Path) -> Process {
     let mut child = Command::new("wasmtime")
         .args([
             "run",
+            // Without a disk cache: the Nix sandbox has no writable home folder.
+            "-C",
+            "cache=n",
             "-S",
             "inherit-network=y",
             "--env",